Is it permissible for the paternal aunt to perform the aqiqah for her brother's daughter, who is seven years and three months old, and would this aqiqah be sufficient?

If the father of the girl is alive and has not performed the Aqiqah for his daughter, then he is responsible for it, and it is permissible for the sister to perform the Aqiqah on her behalf with his permission. However, if the father refuses or is unable to be reached, there is no impediment for the sister to perform the Aqiqah for the girl.